Fix main class resolution failure when loaded from config cache

See gh-22922

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/ResolveMainClassName.java

@@ -28,7 +28,9 @@ import org.gradle.api.DefaultTask;
 import org.gradle.api.InvalidUserDataException;
 import org.gradle.api.Project;
 import org.gradle.api.Task;
+import org.gradle.api.Transformer;
 import org.gradle.api.file.FileCollection;
+import org.gradle.api.file.RegularFile;
 import org.gradle.api.file.RegularFileProperty;
 import org.gradle.api.plugins.BasePlugin;
 import org.gradle.api.plugins.Convention;
@@ -136,19 +138,7 @@ public class ResolveMainClassName extends DefaultTask {
 	}
 
 	Provider<String> readMainClassName() {
-		return this.outputFile.map((file) -> {
-			if (file.getAsFile().length() == 0) {
-				throw new InvalidUserDataException(
-						"Main class name has not been configured and it could not be resolved");
-			}
-			Path output = file.getAsFile().toPath();
-			try {
-				return new String(Files.readAllBytes(output), StandardCharsets.UTF_8);
-			}
-			catch (IOException ex) {
-				throw new RuntimeException("Failed to read main class name from '" + output + "'");
-			}
-		});
+		return this.outputFile.map(new ClassNameReader());
 	}
 
 	static TaskProvider<ResolveMainClassName> registerForTask(String taskName, Project project,
@@ -189,4 +179,23 @@ public class ResolveMainClassName extends DefaultTask {
 		}
 	}
 
+	private static final class ClassNameReader implements Transformer<String, RegularFile> {
+
+		@Override
+		public String transform(RegularFile file) {
+			if (file.getAsFile().length() == 0) {
+				throw new InvalidUserDataException(
+						"Main class name has not been configured and it could not be resolved");
+			}
+			Path output = file.getAsFile().toPath();
+			try {
+				return new String(Files.readAllBytes(output), StandardCharsets.UTF_8);
+			}
+			catch (IOException ex) {
+				throw new RuntimeException("Failed to read main class name from '" + output + "'");
+			}
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/AbstractBootArchiveIntegrationTests.java

@@ -198,6 +198,8 @@ abstract class AbstractBootArchiveIntegrationTests {
 			Attributes mainAttributes = jarFile.getManifest().getMainAttributes();
 			assertThat(mainAttributes.getValue("Start-Class")).isEqualTo("com.example.main.CustomMainClass");
 		}
+		assertThat(this.gradleBuild.build(this.taskName).task(":" + this.taskName).getOutcome())
+				.isEqualTo(TaskOutcome.UP_TO_DATE);
 	}
 
 	private void copyMainClassApplication() throws IOException {